Not really wired up to anything but helps reduce diffs -- driver is
responsible for making decisions based on it, corresponding to
Linux's built-in `threaded irq'.
Nothing special needed here; NetBSD mutex(9) at IPL_* giving spin
locks already blocks `BH' (i.e., softints -- although I'm not sure
softints are relevant to the one use of this in vmwgfx).
Access to the global constty variable is coordinated as follows:
1. Setting constty to nonnull, with atomic_store_release, is allowed
only under the new adaptive constty_lock in thread context. This
serializes TIOCCONS operations and ensures unlocked readers can
safely use a constty pointer read with atomic_load_consume.
2. Changing constty from nonnull to null, with atomic_cas_ptr, is
allowed in any context -- printf(9) uses this to disable a broken
constty.
3. Reading constty under constty_lock is allowed with
atomic_load_relaxed, because while constty_lock is held, it can
only be made null by some other thread/CPU, never made nonnull.
4. Reading constty outside constty_lock is allowed with
atomic_load_consume in a pserialize read section -- constty is
only ever made nonnull with atomic_store_release, in (1).
ttyclose will wait for all these pserialize read sections to
complete before flushing the tty.
5. To continue to use a struct tty pointer in (4) after the
pserialize read section has completed, caller must use tty_acquire
during the pserialize read section and then tty_release when done.
ttyclose will wait for all these references to drain before
returning.
These access rules allow us to serialize TIOCCONS, and safely destroy
ttys, without putting any locks on the access paths like printf(9)
that use constty. Once we set D_MPSAFE, operations on /dev/console
will contend only with other users of the same tty as constty, which
will be an improvement over contending with all other kernel lock
users in the system.
Changes second time around:
- Fix initialization of ok in cons.c cn_redirect.
- Fix reversed sense of conditional in subr_prf.c putone.
Changes third time around:
- Initialize ttyref_cv so we don't panic when trying to use it,
leading to infinite loop when panic tries to take tty_lock to print
the panic message while we already hold tty_lock.
(meaningless macro in mdoc) followed by the first line of the
real BUGS section somehow ended up inserted after the first
word of the STANDARDS section. Delete the noise.
andvar